A generator apparatus that has a generator, a generator controller, and a rectifier bridge is already known. It is used, for example, in a motor vehicle in order to rectify the motor-vehicle generator voltage. The rectifier bridge of the known generator apparatus has at least six Zener diodes, which can be pressed into a heat sink. The purpose of Zener diodes, which become conductive in the reverse direction above a certain voltage, is to protect the generator controller and the vehicle electrical system from overvoltages that are caused by load shutoffs. Zener diodes of different current classes, for example 35A, 50 A, 65 A, and 80 A, are used depending on the rated generator current present in each case. For technical reasons, especially in order to account for a load-dump situation, the aforesaid current classes are further subdivided into so-called Zener voltage classes, for example in 1.5-volt steps between 19 V and 25 V.
The Zener diodes are operated in the reverse direction when large generator currents are abruptly shut off. This is the case, for example, with a so-called load dump, which corresponds to a deenergization of the B+ cable, i.e. of the DC supply voltage lead of the vehicle electrical system. In a load dump situation, the generator controller cannot compensate sufficiently quickly for the shutoff of the generator current. The generator current must continue to flow for physical reasons, since a current cannot jump through an inductance.
Because the loads of the vehicle electrical system, as well as the vehicle battery, are no longer connected to the generator in the case of a load dump, the energy stored in the exciter winding and in the rotor causes a rise in the phase voltages in the phase windings or stator windings. This voltage rise in the phase windings in turn results in a breakdown of the Zener diodes in the reverse direction.
DE 101 35 168 discloses an apparatus for protecting electronic components from overvoltages in the operating voltage, which apparatus has at least one semiconductor transistor that can be made conductive by way of a control circuit at predetermined overvoltage values, with the result that a conversion of electrical energy into thermal energy occurs.